VRML or Virtual Reality Modeling Language could be described as a three dimensional version of HTML, a page description language. VRML describes the objects and environments of webspace. Don't let the term 'virtual reality' mislead you, there are no head mounted displays or data gloves, yet. VR simply means that these environments are interactive and can be manipulated in real-time. You can walk or fly around, turn your head, and click objects. Hyper linked objects can be attached to html pages, other vrml models, sound files, or any other available media. All you need to explore these worlds is a vrml browser.
For more information about vrml including a short documented history, see the VRML Forum at (Wired)
Netscape, Mosaic, etc. are html browsers. They allow you to skip from page to page across the internet. A vrml browser is very similar, in fact it can run inside your html browser. Anytime you click on a link to a vrml world (.wrl) the vrml browser is launched and the world is loaded. It can also run as a separate application.
There are many available free on the web from several companies. Visit the VRML Repository for a full list of browsers available for many platforms.